Adam and Eve did not have 2 children. If you read the story about Adam and Eve from the time they sinned against God you will understand why the bible only mentioned the sons that Adam and Eve had.
The reason the sons of Adam and Eve were mentioned only was this: At the time Adam and Eve sinned against God, God told Adam and Eve that, I will put enmity between thee and the woman, and between thy seed and her seed, it shall bruise thy head, and thou shall bruise his heel (gen 3:15). So the seed(son) which is to bruise the serpent's head(Satan) was Jesus. Jesus was the seed to bruise the serpent's head. So Adam and Eve thought that they were the ones to give birth to Jesus. So each time Eve given birth to a son, she said God hath appointed her another SEED (Gen 4:1, gen 4:25). Further in the scriptures you will see where cain and his wife begotten sons and daughter. So for cain to have a wife, eve must have given birth to daughters. Further more, 2 male cannot give birth to a child. Simple.